This invention relates to low molecular weight polyurethane resins for use in laminating inks displaying high lamination bond strength.
With the increasing use of versatile flexible packagings, there has arisen a need to provide high performance printing inks, various coating agents, and adhesives that are applied for decorative, surface protecting, or other purposes. For example, the printing inks for use on plastic films are required to exhibit much better performance than conventional versions in various aspects such as printability, adhesion to various kinds of films, antiblocking properties, and gloss.
Particularly in the food packaging industry, laminated flexible packagings are currently used since direct contact of the contents with inks must be avoided to ensure utmost hygiene while impressing consumers with the high quality of printing. Lamination is carried out by two methods described hereunder: extrusion lamination which comprises printing an ink on various plastic film bases, applying a primer coat on the printed surface as required, and then laminating a molten layer of polyolefin or the like; and adhesive lamination which comprises applying an adhesive to the printed surface, and then laminating a plastic film. Whichever method is adopted, inks to be used on the various kinds of plastic films to be laminated are required to adhere strongly not only to the base film but also to the film to be laminated.
Solvent based inks represent the bulk of the laminating inks consumed for packaging. The main resins used in solvent applications are acrylics, modified acrylics, polyamides, and urethanes. These soluble resins typically wet out and adhere to the film surfaces and laminated structures with superior bonds.
Water based inks are gaining in popularity as a way to reduce organic emissions and retained solvent in food packaging. In water, acrylics, and more recently urethanes, have found utility and are of the solution and latex type. In comparison with solvent based inks, water based inks suffer from wettability, adhesion, and bond strength limitations
Japanese Patent Application No. 354568/1991 discloses an aqueous laminating printing ink for use as a binder in an aqueous polyurethane resin containing a polycarbonate diol as a diol component and which was improved in adhesion to various plastic films and peel strength of laminates. Flexible packagings produced by using such aqueous laminating inks can be used to make bags for packaging dry foods.
Japanese Patent Application No. 317425/1992 discloses a method for improving the adhesion to various plastic films and the peel strength of laminates by using, as an ink binder resin, an acrylic copolymer that had functional groups capable of reaction with a hydrazine group or a hydrazide group introduced into the molecule, and hydrazinc compounds as a crosslinking agent, said agent with those functional groups and carbonyl groups that developed on the film surface by subsequent surface treatment. However, compared to inks using polyurethane-base binder resins that can be used and, hence, the printing inks used in this method are poor in pigment dispersability and printability. Furthermore, during storage, the binder resins will crosslink with the crosslinking agents to lower their fluidity and capability for redissolution. As another problem, there is no guarantee for the occurrence of positive crosslinking said agents with the binder resins and the film surface, and this has often caused nonuniformity in adhesion and the peel strength of laminates.
U.S. Pat. No. 5,656,701 discloses polyurethane resins having at least one group selected from hydrazine groups, hydrazide groups and semicarbazide groups (the group of these functional groups is described as HYD groups), in the molecule, a process for producing the same, aqueous ink compositions for plastic film using the same as binders, aqueous adhesive agents for film lamination and a method of making laminates using said ink and/or adhesives. While the polyurethane resins described in this patent posses better than before physical properties, the extrusion lamination bond strength was still considered to be relatively weak.
The present invention provides a polyurethane resin having a number average molecular weight of up to 5,000, wherein the resin is prepared by:
(a) reacting a diisocyanate compound with at least one monomer diol compound and optionally a polymer diol compound without a chain extender to form a polymer, wherein the molar ratio of the monomer diol compound over the polymer diol compound when present is above 1; and
(b) capping the polymer with a terminator.
The present invention also provides a method for preparing a polyurethane resin comprising:
(a) reacting a diisocyanate compound with at least one monomer diol compound and optionally a polymer diol compound without a chain extender to form a polymer, wherein the molar ratio of the monomer diol compound over the polymer diol compound when present is above 1; and
(b) capping the polymer with a terminator.
Other objects and advantages of the present invention will become apparent from the following description and appended claims.
It has now been surprisingly found that polyurethane resins of the present invention can be used in laminating inks displaying high lamination bond strength when such resins have a number average molecular weight of up to 5,000 and prepared by reacting an isocyanate compound with at least one monomer diol compound and optionally a polymer dial compound without a chain extender to form a polymer, wherein the molar ratio of the monomer diol compound over the polymer diol compound when present is above 1, followed by capping the polymer with a terminator.
Specifically, when such resins were formulated as the major resin in an aqueous laminating ink, the resultant extrusion lamination bond strengths on a wide variety of plastic films were strikingly superior to those displayed by a similar standard not containing the polyurethane resins of the present invention. These results, particularly on a flexible type of substrate were especially surprising in view of the expected rigidity of the resins.
Preferably, the resin of the present invention have an average molecular weight of about 3,000 to 5,000. Also preferably, the monomer diol compound is preferably dimethylol propionic acid, 2-methyl-1,3-propanediol, chloroglycerol, 1,4-butanediol, 1,6-hexanediol or neopentylglycol. The organic diisocyanate may be any monomeric diisocyanate but is more preferably toluenediisocyanate.
The polymer diol may be one of any available examples but is preferably Terathane 650 or Tone 0200, and the terminator is a C1 to C4 primary alcohol such as propanol, or Abitol, or hydroxylated surfactants such as Igepal C-720.
The following Table 1 illustrates the length of the various diols that can be used in preparing the resins of the present invention:
Also preferably, the molar ratio of the monomer diol compound over the polymer diol compound when present in the resin of the present invention is above 2. The resin can also have acid number of 0 to about 300, preferably, of about 60 too 140, and more preferably about 120 to 135.
Preferably, the formation of the polymer of the present invention prior to capping (or termination) is carried out in an organic solvent, followed by addition of water and a basic compound and removal of the organic solvent by azeotropic distillation. Preferably, the organic solvent is an aprotic compound having a boiling temperature of about 60 to 140xc2x0 C., more preferably, methylethylketone. Also preferably, the basic compound is ammonium hydroxide.